If you are speaking of the image
https://tagungszentrum-karimu.de/wp-content/uploads/2015/11/2.jpg
this image is only 1600px wide, which is why it is not full width in an 1800px wide browser.
The best solution to get it full width all the time would be to use a larger image.
You could force it to always go 100% wide as it is now but when the browser is larger than the image this will result in a loss of image quality.
To do that you could simply add a width:100% in a style attribute of the image tag
<img style=”width:100%;” …../>